Recently, vehicle-installed radar devices, employing laser light and being for lightening the burden placed on a driver, improving convenience, and improving safety, are being developed. With such a vehicle-installed radar apparatus, laser light emitted from a light emitting element is illuminated, by a polygon mirror having a plurality of reflecting angles, in a plurality of directions spanning at least the interior of a predetermined angular range in a vehicle width direction. The vehicle-installed radar apparatus detects reflected light of each laser light by a photodetecting circuit and detects a reflecting object based on a time between a timing of illumination of laser light and a timing of photodetection of reflected light.
The photodetecting circuit used in the vehicle-installed laser apparatus is required to be high in detection precision and, for example, Patent Document 1 discloses a photodetecting circuit that is improved in detection sensitivity of reflected light reflected from a reflecting object. With the photodetecting circuit described in Patent Document 1, a predetermined number of photodetecting signals that are output based on a predetermined number of laser beams, which are illuminated adjacently, are summed and a summed signal is output. By summing a predetermined number of photodetecting signals, the photodetecting signal component corresponding to the reflected light from the reflecting object is amplified. It is thus deemed that the sensitivity of detection of the reflected light from the reflecting object is improved.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2004-177350